In a production of semiconductor circuit elements, workpieces in the form of semiconductor wafers are generally coated with a photosensitive layer and are exposed by an appropriate optical system effecting image reduction in a pattern determined by a mask so that subsequent development and treatment may establish certain conductive or nonconductive paths on the wafer and/or from circuit elements thereon.
In many cases, circuit elements have already been formed on the wafer before such exposure so that during the exposure a precise positioning of the semiconductor wafer relative to the mask is essential for proper correlation (overlay) of successively formed circuit patterns.
To effect this correlation it is known to align the wafer by imaging into each other corresponding alignment marks on the mask and alignment patterns on the wafer by means of alignment light reflected from the water. If the alignment patterns on the wafer are related to the wafer as a whole and not to its individual fields this process is known as global alignment. Following global alignment the wafer is displaced in two orthogonal directions (X,Y) to bring the next field on the wafer into proper position for exposure after exposure of another field has been completed. The amount by which the wafer is displaced in either direction is predetermined, it corresponds to the distance between the individual fields which is supposed to be known. In order to correct the error due to the difference between an actual distance of the fields and its assumed value sophisticated devices incorporate means for the alignment of each individual field of the wafer after is has been brought into an approximately correct position by global alignment and "blind" diplacement in steps of predetermined lengths. For this purpose field alignment marks are associated with every field on the wafer which make it possible to correct the position of the field in cartesian and angular coordinates X, Y and .phi. and in the direction of the optical axis.
The methods described so far can be improved if a way is found to make the predetermined value of the steps, which bring each field into approximate alignment, as ideal as possible. The main reason why this predetermined value needs correction seems to be that the length of the steps required to bring subsequent fields on a wafer under the object lens is found to change, if a certain wafer is repeatedly exposed. This happens not only if different machines are used for successive printing processes but also if a single machine has changed its dimensions, e.g. because of temperature changes. A method has to be found to take into consideration the difference of scale between the machines used for a previous exposure of a wafer and the exposure to be effected. Assuming that any number of wafers prepared in a uniform process on a single machine shows an identical array of fields the different scale of the processing equipment can be compensated by measuring for one of the wafers the length of the steps in X- and Y-direction, which are necessary to reach coincidence of the field alignment marks with the marks on the mask and to use the length of the steps with thus has been found by measurement for one wafer for the "blind" stepping of all subsequent wafers.
Applicant applied the scaling procedure just described and observed that it makes it possible to bring one of the field alignment marks associated with the different fields on the wafer into proper alignment. However, a rotational alignment error remained, i.e. the pattern on the mask and the pattern on the corresponding field on the wafer had slightly different directions. So far this error had not attracted much attention and the only measure to minimize it had been a rotation of the wafer as a whole during global alignment.
Upon close inspection of the problem it was found that the rotational alignment error can only be reduced but cannot be eliminated by global alignment, because it differs for the different fields on the surface of a single wafer. At first it was thought that the deviations are erratic, as there is no obvious regularity in their appearance. This would mean that the rotational alignment error remaining after global rotational alignment could only be corrected separately for every field. Surprisingly it was then found that the seemingly erratic rotational alignment errors appear almost identically on every wafer out of a batch that was produced on a single machine. This makes it possible to choose one out of the identical batch of wafers, measure the rotational adjustment necessary to bring each field into the correct direction in the horizontal plane and to use the measured value for an automatic rotational adjustment of all other wafers of the batch.
As explained the invention is based on the insight that the fields on any wafer have slightly differing orientations but that corresponding fields on two different wafers are equally directed provided the wafers have been produced in a uniform process on a single device. This makes it possible to prepare one out of a batch of otherwise identical wafers in such a way that the orientation of its fields may be easily checked. In this sense it is advantageous to leave one wafer without a photosensitive cover which allows for fast detection of the marks on its surface. Of course, further processing of this master wafer would only be possible if it was subsequently covered with a photosensitive layer.
The success of the inventive method proves that there is a systematic cause for the rotational deviations among the printed fields of a wafer. An assumption concerning the nature of this cause shall be discussed in connection with the drawings.